1. Field of the Invention
The invention relates to a process for producing diesel fuel from bitumen and gas conversion. More particularly, the invention relates to a process in which a gas conversion process produces steam, naphtha and a diesel fraction, with the steam used for bitumen production, the naphtha for bitumen pipelining and the bitumen converted to produce a diesel fraction. The two different diesel fractions are mixed to form a diesel fuel stock.
2. Background of the Invention
Very heavy crude oil deposits, such as the tar sand formations found in places like Canada and Venezuela, contain trillions of barrels of a very heavy, viscous petroleum, commonly referred to as bitumen. The bitumen has an API gravity typically in the range of from 5xc2x0 to 10xc2x0 and a viscosity, at formation temperatures and pressures that may be as high as a million centipoise. The hydrocarbonaceous molecules making up the bitumen are low in hydrogen and have a resin plus asphaltenes content as high as 70%. This makes the bitumen difficult to produce, transport and upgrade. Its viscosity must be reduced in-situ underground for it to be pumped out (produced), it needs to be diluted with a solvent if it is to be transported by pipeline to an upgrading or other facility, and its high resin and asphaltene content tends to produce hydrocarbons low in normal paraffins. As a consequence, diesel fuel produced from bitumen tends to be low in cetane number and a higher cetane hydrocarbon must be blended with it. Thus, producing a diesel fraction from bitumen requires a plentiful supply of (i) steam, most of which is not recoverable, (ii) a diluent which can be used preferably on a once-through basis and (iii) a high cetane diesel fraction for blending with the low cetane bitumen diesel fraction.
A process for producing a diluent for transporting the bitumen upgrading facilities by pipeline is disclosed, for example, in U.S. Pat. No. 6,096,192.
Gas conversion processes, which produce hydrocarbons from a synthesis gas derived from natural gas, are well known. The synthesis gas comprises a mixture of H2 and CO, which are reacted in the presence of a Fischer-Tropsch catalyst to form hydrocarbons. Fixed bed, fluid bed and slurry hydrocarbon synthesis processes have been used, all of which are well documented in various technical articles and in patents. Both light and heavy hydrocarbons may synthesized, including low viscosity naphtha fractions and diesel fractions relatively high in cetane number. These processes also produce steam and water. It would be an improvement to the art if bitumen production and gas conversion could be integrated, to utilize products of the gas conversion process to enhance bitumen production and transportation, and to produce a diesel fraction having a cetane number higher than a diesel fraction produced from the bitumen.
The invention relates to a process in which a hydrocarbon gas is converted to a synthesis gas feed, from which liquid hydrocarbons, including naphtha and diesel fractions are synthesized and steam is generated, to facilitate bitumen production and transportation and to improve the cetane number of diesel produced by upgrading the bitumen. The conversion of a hydrocarbon gas, and preferably natural gas to synthesis gas, and the synthesis or production of hydrocarbons from the synthesis gas will hereinafter be referred to as xe2x80x9cgas conversionxe2x80x9d. The conversion of natural gas to synthesis gas and the synthesizing of hydrocarbons from the synthesis gas are achieved by any suitable synthesis gas and hydrocarbon synthesis processes. At least the higher boiling portion of the diesel fraction produced by the gas conversion is hydroisomerized to reduce its pour point, while preserving cetane number. The diesel fraction produced by the bitumen conversion is hydrotreated to reduce its heteroatom, aromatics and metals contents. The preferably natural gas used to produce the synthesis gas will typically and preferably come from the bitumen field or a nearby gas well. The synthesis gas is produced by any suitable process. The gas conversion process produces liquid hydrocarbons, including naphtha and diesel fractions, steam and water. The steam is used to stimulate the bitumen production, the naphtha is used to dilute the bitumen for transportation by pipeline to upgrading, and the higher cetane, hydroisomerized diesel is blended with the lower cetane bitumen diesel, to produce a diesel fuel stock. Thus, the invention broadly relates to an integrated gas conversion and bitumen production and upgrading process, in which gas conversion steam, naphtha and diesel fraction hydrocarbon liquids are respectively used to stimulate bitumen production, dilute the bitumen for pipelining and upgrade a bitumen-derived diesel fraction.
Synthesis gas comprises a mixture of H2 and CO and, in the process of the invention, it is contacted with a suitable hydrocarbon synthesis catalyst, at reaction conditions effective for the H2 and CO in the gas to react and produce hydrocarbons, at least a portion of which are liquid and include the naphtha and diesel fractions. It is preferred that the synthesized hydrocarbons comprise mostly paraffinic hydrocarbons, to produce a diesel fraction high in cetane number. This may be achieved by using a hydrocarbon synthesis catalyst comprising a cobalt and/or ruthenium catalytic component, and preferably at least cobalt. At least a portion of the gas conversion synthesized diesel fraction is upgraded by hydroisomerization to lower its pour and freeze points. The higher boiling diesel hydrocarbons (e.g., 500-700xc2x0 F.) are highest in cetane number and are preferably hydroisomerized under mild conditions, to preserve the cetane number. The gas conversion portion of the process produces high and medium pressure steam, all or a portion of which are injected into the ground to stimulate the bitumen production. Water is also produced by the hydrocarbon synthesis reaction, all or a portion of either or both of which may be heated to produce steam for the bitumen production. Thus, by xe2x80x9cgas conversion steamxe2x80x9d or xe2x80x9csteam obtained or derived from a gas conversion processxe2x80x9d in the context of the invention is meant to include any or all of the (i) high and medium pressure steam produced by the gas conversion process and (ii) steam produced from heating the hydrocarbon synthesis reaction water, and any combination thereof. By bitumen production is meant steam stimulated bitumen production, in which steam is injected down into a bitumen formation, to soften the bitumen and reduce its viscosity, so that it can be pumped out of the ground. While the naphtha diluent may be recovered from the diluted bitumen after transportation, it is preferred that the naphtha diluent be used on a once-through basis and not be recycled back to bitumen dilution. In another embodiment of the invention, hydrogen is produced from the synthesis gas. This hydrogen may be used for hydroisomerizing the gas conversion diesel fraction to reduce its pour point and, if the bitumen upgrading facility is close, for bitumen upgrading. The hydrocarbon synthesis reaction also produces a tail gas that contains methane and unreacted hydrogen. This tail gas may be used as fuel to produce steam for bitumen production, boiler water, pumps or other process utilities.
Upgrading bitumen in the process of the invention comprises fractionation and two or more conversion operations, including hydroconversion in which hydrogen is present as a reactant, to produce and upgrade the diesel fraction. By conversion is meant at least one operation in which at least a portion of the molecules is changed. Bitumen conversion comprises catalytic or non-catalytic cracking, and hydroprocessing operations such as hydrocracking, hydrotreating and hydroisomerization, in which hydrogen is a reactant. Coking is more typically used for the cracking and cracks the bitumen into lower boiling material and coke, without the presence of a catalyst. At least a portion of these lower boiling hydrocarbons, including the hydrocarbons boiling in the diesel fuels range, are hydrotreated to reduce the amount of, heteroatoms (e.g., sulfur and nitrogen), aromatics, including condensed aromatics and metals that may be present.
The process of the invention briefly comprises (i) stimulating the production of bitumen with steam obtained from a hydrocarbon gas and preferably a natural gas fed gas conversion process that produces naphtha and diesel hydrocarbon fractions and steam, (ii) diluting the produced bitumen with naphtha produced by the gas conversion to form a pipelineable fluid mixture comprising the bitumen and diluent, (iii) transporting the mixture by pipeline to a bitumen upgrading facility, (iv) upgrading the bitumen to form lower boiling hydrocarbons, including a diesel fraction, and (v) forming a mixture of the gas conversion and bitumen diesel fractions. In a more detailed embodiment the invention comprises the steps of (i) stimulating the production of bitumen with steam obtained from a natural gas fed gas conversion process that produces naphtha and diesel hydrocarbon fractions and steam, (ii) treating at least a portion of the gas conversion diesel fraction to reduce its pour point, (iii) diluting the produced bitumen with naphtha produced by the gas conversion, to form a pipelineable fluid mixture comprising the bitumen and diluent and transporting the mixture by pipeline to a bitumen upgrading facility, (iv) upgrading the bitumen to form lower boiling hydrocarbons, including a diesel fraction and (v) treating the bitumen diesel fraction to reduce its sulfur content. At least a portion of both treated diesel fractions is combined to form a diesel stock having a cetane number higher than that of the treated bitumen diesel fraction. In a still more detailed embodiment the process of the invention comprises:
(i) converting natural gas to a hot synthesis gas comprising a mixture of H2 and CO which is cooled by indirect heat exchange with water to produce steam;
(ii) contacting the synthesis gas with a hydrocarbon synthesis catalyst in one or more hydrocarbon synthesis reactors, at reaction conditions effective for the H2 and CO in the gas to react and produce heat, liquid hydrocarbons including naphtha and diesel fuel fractions, and a gas comprising methane and water vapor;
(iii) removing heat from the one or more reactors by indirect heat exchange with water to produce steam;
(iv) hydroisomerizing at least a portion of the diesel fraction formed in (ii) to reduce its pour point;
(v) passing at least a portion of the steam produced in either or both steps (i) and (iii) into a tar sand formation to heat soak and reduce the viscosity of the bitumen;
(vi) producing the bitumen by removing it from the formation;
(vii) reducing the viscosity of the produced bitumen by mixing it with a diluent comprising at least a portion of the naphtha produced in step (ii);
(viii) transporting the mixture by pipeline to a bitumen upgrading facility;
(ix) upgrading the bitumen to lower boiling hydrocarbons, including a diesel fuel fraction containing heteroatom compounds;
(x) hydrotreating the bitumen diesel fuel fraction to reduce its heteroatom content, and
(xi) combining at least a portion of the pour point reduced and hydrotreated diesel fuel fractions.
The hydrotreating also reduces the amount of unsaturated aromatic and metal compounds. By bitumen diesel fraction, referred to above, is meant a diesel fuel fraction produced by upgrading the bitumen including coking and fractionation. The tar sand formation is preferably an underground or subterranean formation having a drainage area penetrated with at least one well, with the softened and viscosity-reduced bitumen produced by removing it from the formation up through the well.